Many tamper-proof caps are known in the art which are designed to demonstrate to a user whether or not a cap has previously been removed. The most common tamper-proof cap is a screw-on lid, the lower lip of which is attached to a collar via a frangible element. The collar is prevented from rotating with the cap so that, when the cap is rotated, the frangible elements break to separate the collar from the lid thereby providing a visual indication that the cap has previously been opened.